<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="../reset.css">
    <style>
        .box1{
            width: 800px;
            height: 800px;
            background-color: #bfa;
            overflow: hidden;
        }
        .box1 div{
            width: 100px;
            height: 100px;
            margin-bottom: 100px;
        }

        .box2{
            background-color: #4f584d;
            /*过渡
            当一个属性发生变化时，需要一定的时间
            */
            transition: all 2s;
        }

        .box1:hover .box2{
            width: 200px;
            height: 200px;
        }
    </style>
</head>
<body>
<div class="box1">
    <div class="box2"></div>
</div>
</body>
</html>